I was at Yellowstone several years ago and at the time my max length was 300mm. I had a lot of good pictures, but I could have had a lot more with a longer length, myself, I would go with the 200-800. I saw a lot of wildlife that was pretty far away, and no way to get closer. I saw bighorn sheep that were on ledges across canyons, grizzly bears high up on hills. And if you want to see wolves, they will be quite a distance as they try to avoid people. Wildlife is everywhere in Yellowstone so you want to keep a long lens on your camera. But you will also want to have wide angle and normal lenses too for some of the volcanic features of Yellowstone. and even some wildlife. We were not in the park very long before there was a "buffalo jam", the buffalo decide they are going to be on the road, so you will wait til they are gone. I was driving and my son had my camera, tried to shoot a buffalo walking past the car, but it was too close to focus with a telephoto. I would suggest having two cameras with you if can, maybe rent a second.
I'll add, get up early and be in the park when the sun rises, wildlife is most active in the morning and evening. There is wildlife everywhere, but especially along the river valleys. I spent time along the Madison River and the Yellowstone River and saw the most wildlife there. Most of the volcanic feature are pretty good too, the only bad one was Old Faithful and it is overrun with tourists. Be alert because there as may be wildlife on the roads and people that drive that are preoccupied (I guess that's everywhere). Have fun and get some great photos!
